Ink jet printing has become recognized as a prominent contender in the digitally controlled, electronic printing arena because, e.g., of its non-impact, low-noise characteristics, its use of plain paper and its avoidance of toner transfer and fixing. Ink jet printing mechanisms can be categorized by technology as either drop on demand ink jet (DOD) or continuous ink jet (CIJ).
The first technology, “drop-on-demand” (DOD) ink jet printing, provides ink drops that impact upon a recording surface using a pressurization actuator, for example, a thermal, piezoelectric, or electrostatic actuator. One commonly practiced drop-on-demand technology uses thermal actuation to eject ink drops from a nozzle. A heater, located at or near the nozzle, heats the ink sufficiently to boil, forming a vapor bubble that creates enough internal pressure to eject an ink drop. This form of inkjet is commonly termed “thermal ink jet (TIJ).”
The second technology commonly referred to as “continuous” ink jet (CIJ) printing, uses a pressurized ink source to produce a continuous liquid jet stream of ink by forcing ink, under pressure, through a nozzle. The stream of ink is perturbed using a drop forming mechanism such that the liquid jet breaks up into drops of ink in a predictable manner. One continuous printing technology uses thermal stimulation of the liquid jet with a heater to form drops that eventually become print drops and non-print drops. Printing occurs by selectively deflecting one of the print drops and the non-print drops and catching the non-print drops. Various approaches for selectively deflecting drops have been developed including electrostatic deflection, air deflection, and thermal deflection.
Printing systems that combine aspects of drop-on-demand printing and continuous printing are also known. These systems, often referred to as flow through liquid drop dispensers, provide increased drop ejection frequency when compared to drop-on-demand printing systems without the complexity of continuous printing systems.
Micro-Electro-Mechanical Systems (or MEMS) devices are becoming increasingly prevalent as low-cost, compact devices having a wide range of applications. As such, MEMS devices, for example, MEMS transducers, have been incorporated into both DOD and CIJ printing mechanisms.
MEMS transducers include both actuators and sensors that convert an electrical signal into a motion or they convert a motion into an electrical signal, respectively. Typically, MEMS transducers are made using standard thin film and semiconductor processing methods. As new designs, methods and materials are developed, the range of usages and capabilities of MEMS devices is be extended.
MEMS transducers are typically characterized as being anchored to a substrate and extending over a cavity in the substrate. Three general types of such transducers include a) a cantilevered beam having a first end anchored and a second end cantilevered over the cavity; b) a doubly anchored beam having both ends anchored to the substrate on opposite sides of the cavity; and c) a clamped sheet that is anchored around the periphery of the cavity. Type c) is more commonly called a clamped membrane, but the word membrane will be used in a different sense herein, so the term clamped sheet is used to avoid confusion.
Sensors and actuators can be used to sense or provide a displacement or a vibration. For example, the amount of deflection δ of the end of a cantilever in response to a stress σ is given by Stoney's formulaδ=3σ(1−ν)L2/Et2  (1),where ν is Poisson's ratio, E is Young's modulus, L is the beam length, and t is the thickness of the cantilevered beam. In order to increase the amount of deflection for a cantilevered beam, one can use a longer beam length, a smaller thickness, a higher stress, a lower Poisson's ratio, or a lower Young's modulus. The resonant frequency of vibration is given byω0=(k/m)1/2,  (2),where k is the spring constant and m is the mass. For a cantilevered beam, the spring constant k is given byk=Ewt3/4L3  (3),where w is the cantilever width and the other parameters are defined above. For a lower resonant frequency one can use a smaller Young's modulus, a smaller width, a smaller thickness, a longer length, or a larger mass. A doubly anchored beam typically has a lower amount of deflection and a higher resonant frequency than a cantilevered beam having comparable geometry and materials. A clamped sheet typically has an even lower amount of deflection and an even higher resonant frequency.
Thermal stimulation of liquids, for example, inks, ejected from DOD printing mechanisms using a heater or formed by CIJ printing mechanisms using a heater is not consistent when one liquid is compared to another liquid. Some liquid properties, for example, stability and surface tension, react differently relative to temperature. As such, liquids are affected differently by thermal stimulation often resulting in inconsistent drop formation which reduces the numbers and types of liquid formulations used with DOD printing mechanisms or CIJ printing mechanisms.
Accordingly, there is an ongoing need to provide liquid ejection mechanisms and ejection methods that improve the reliability and consistency of drop formation on a liquid by liquid basis while maintaining individual nozzle control of the mechanism in order to increase the numbers and types of liquid formulations used with these mechanisms. There is also an ongoing effort to increase the reliability and performance of flow through liquid drop dispensers.